Pre-Installation Planning
You'll need Rubygems >= 1.3.0, which isn't in the version of ports that ships with 7.1-RELEASE, so upgrade ports first.
Upgrading Ports
Install Ruby and Rubygems
Make sure you have Ruby and Rubygems installed. On FreeBSD with ports installed:
Installing Ruby and Rubygems
Proceed to Installation with RubyGems to add the Opscode Gem Source and Install Chef and Ohai.